Make approve method compliant with ERC20

Author: jakub-wojciechowski

contracts/token/StandardToken.sol

@@ -44,13 +44,6 @@ contract StandardToken is ERC20, BasicToken {
    * @param _value The amount of tokens to be spent.
    */
   function approve(address _spender, uint256 _value) public returns (bool) {
-
-    // To change the approve amount you first have to reduce the addresses`
-    //  allowance to zero by calling `approve(_spender, 0)` if it is not
-    //  already 0 to mitigate the race condition described here:
-    //  https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
-    // require((_value == 0) || (allowed[msg.sender][_spender] == 0));
-
     allowed[msg.sender][_spender] = _value;
     Approval(msg.sender, _spender, _value);
     return true;